As is well known, in wavelength division multiplexing (WDM) optical communications used in recent large-capacity long-distance optical transmission systems, level deviations between channels lead to deterioration of signals.
Correspondingly, optical amplifiers used in long-distance transmission such as submarine optical cables must have flat and wide-band gain wavelength characteristics in addition to conventional low noise and high efficiency.
This makes evaluation of the wavelength characteristics of an optical fiber amplifier important.
An optical fiber amplifier is of course a kind of an amplifier. Therefore, it is necessary to measure a gain G indicated by the ratio of a light intensity PIN of an input optical signal to a light intensity POUT of an output optical signal.
As is well known, owing to its light amplification mechanism, an optical fiber amplifier produces spontaneous emission, even when no optical signal is input to its input terminal, and this spontaneous emission is amplified and output to its output terminal.
This amplified spontaneous emission (ASE) acts as noise with respect to an amplified optical signal.
It is, therefore, important to measure the light intensity PASE of this amplified spontaneous emission (ASE).
As an index indicating the noise resistance of an optical fiber amplifier, a noise figure NF indicated by equation (1) below which includes the measured gain G and light intensity PASE is generally employed: EQU NF=f(G, PASE, .nu., .DELTA..nu.) (1)
where
.nu.: the optical frequency of an input optical signal PA1 G: gain PA1 .DELTA..nu.: the measurement frequency resolution width (measurement frequency width) of a light intensity measurement apparatus
Hence, the characteristics of an optical fiber amplifier are evaluated in terms of the gain G and the noise figure NF.
Conventionally, to evaluate the characteristics of an optical fiber amplifier, in an arrangement as shown in FIG. 10, a laser light source 101 and an optical fiber amplifier 5 are connected to an optical spectrum analyzer 103 via an optical path switch 102.
First, the optical path switch 102 is closed to the laser light source 101. The optical spectrum analyzer 103 obtains the light intensity PIN, shown in FIG. 11, with respect to an optical wavelength .lambda. of an input optical signal to the optical fiber amplifier 5.
Next, the optical path switch 102 is closed to the optical fiber amplifier 5. The optical spectrum analyzer 103 obtains the light intensity POUT, shown in FIG. 11, at the optical wavelength .lambda. of an output optical signal from the optical fiber amplifier 5.
Accordingly, the gain G is calculated by EQU G=POUT/PIN (2)
As shown in FIG. 11, however, the light intensity PASE of amplified spontaneous emission (ASE) is buried in the light intensity POUT of the amplified output optical signal. This makes the light intensity PASE of the amplified spontaneous emission (ASE) difficult to directly measure.
As a method of measuring the light intensity PASE of this amplified spontaneous emission (ASE), a level interpolation method, polarization nulling method, and pulse method have been proposed.
(Explanation of Pulse Method)
Of these three methods, the pulse method (e.g., Jpn. Pat. Appln. KOKAI Publication Nos. 6-224492 and 9-18391) uses the fact that the recovery time to the ground state of rare earth element light of metastable erbium doped in the core of an optical fiber of an optical fiber amplifier is relatively long. In this method, an input optical signal to an optical fiber amplifier is turned on and off at periods shorter than this recovery time, the light intensity POUT of an output optical signal is measured during the ON period, and the light intensity PASE of the amplified spontaneous emission (ASE) is measured during the OFF period.
FIG. 12 illustrates an optical fiber amplifier evaluating apparatus of the prior application using this pulse method.
That is, an optical modulation unit 21 shown in FIG. 12 is proposed in an international application (PCT/JP98/02015) filed by this international applicant.
As depicted in FIG. 12, a light source 201a for outputting a wavelength .lambda.1 is connected to an optical attenuator 202a, a light source 201b for outputting a wavelength .lambda.2 is connected to an optical attenuator 202b, . . . , a light source 201n for optically outputting a wavelength .lambda.n is connected to an optical attenuator 202n.
An optical multiplexer 203 multiplexes, as will be described later, light components from these optical attenuators 202a, 202b, . . . , 202n.
The optical signal multiplexed by this optical multiplexer 203 is input to an optical fiber amplifier 5 via the optical modulation unit 21.
The output optical signal from this optical fiber amplifier 5 is again input to an optical spectrum analyzer 207 via the optical modulation unit 21.
(Measurement of Light Intensity PIN)
A controller 208 switches, as indicated by the dotted lines in FIG. 12, a first optical path switch 28 and a second optical path switch 33 in the optical modulation unit 21. The controller 208 also sends a light intensity measurement command to the optical spectrum analyzer 207.
In this state, as shown in FIG. 13, a first optical modulator 23 in the optical modulation unit 21 modulates the light, that is emitted by the light sources 201a, 201b, . . . , 201n and so wavelength-multiplexed as to have a plurality of wavelengths .lambda.1, .lambda.2, .lambda.3, . . . , .lambda.n-1, .lambda.n, . . . by the optical multiplexer 203, into a rectangular optical signal which is turned on and off at a predetermined period T0 (FIG. 2A).
The optical signal modulated by this first optical modulator 23 is fed into the optical spectrum analyzer 207 via the first optical path switch 28 and the second optical path switch 33.
The optical spectrum analyzer 207 analyzes the spectrum of this incoming light and obtains the light intensity PIN (.lambda.=.lambda.1, .lambda.2, .lambda.3, . . . , .lambda.n-1, .lambda.n, . . . ) at each wavelength .lambda..
The optical spectrum analyzer 207 sends the measured light intensity PIN(.lambda.) to the controller 208.
(Measurement of Light Intensity POUT)
As shown in FIG. 12, the controller 208 sets the first optical path switch 28 in the steady state indicated by the solid lines and the second optical path switch 33 in switched state indicated by the dotted lines, and sends a light intensity measurement command to the optical spectrum analyzer 207.
In this state, as shown in FIG. 13, the first optical modulator 23 in the optical modulation unit 21 modulates the light, that is emitted by the light sources 201a, 201b, . . . , 201n and so wavelength-multiplexed as to have the wavelengths .lambda.1, .lambda.2, .lambda.3, . . . , .lambda.n-1, .lambda.n, . . . by the optical multiplexer 203, into a rectangular optical signal which is turned on and off at the predetermined period T0.
The optical signal modulated by this first optical modulator 23 is fed into the optical fiber amplifier 5 as an object to be measured and is optically amplified.
The amplified optical signal output from this optical fiber amplifier 5 is directly fed into the optical spectrum analyzer 207 via the first optical path switch 28 and the second optical path switch 33 in the optical modulation unit 21.
The optical spectrum analyzer 207 analyzes the spectrum of this incident light and obtains the light intensity POUT (.lambda.=.lambda.1, .lambda.2, .lambda.3, . . . , .lambda.n-1, .lambda.n, . . . ) at each wavelength .lambda..
The optical spectrum analyzer 207 sends the measured light intensity POUT(.lambda.) to the controller 208.
(Measurement of Light Intensity PASE)
As shown in FIG. 12, the controller 208 sets the first optical path switch 28 and the second optical path switch 33 in the steady state indicated by the solid lines, and sends a light intensity measurement command to the optical spectrum analyzer 207.
In this state, as shown in FIG. 13, the first optical modulator 23 in the optical modulation unit 21 modulates the light, that is emitted by the light sources 201a, 201b, . . . , 201n and so wavelength-multiplexed as to have the wavelengths .lambda.1, .lambda.2, .lambda.3, . . . , .lambda.n-1, .lambda.n, . . . by the optical multiplexer 203, into a rectangular optical signal which is turned on and off at the predetermined period T0.
The optical signal modulated by this first optical modulator 23 is fed into the optical fiber amplifier 5 as an object to be measured and is optically amplified.
The amplified optical signal output from this optical fiber amplifier 5 is fed into a second optical modulator 35 via the first optical path switch 28 and the second optical path switch 33 in the optical modulation unit 21.
From the amplified optical signal fed into the second optical modulator 35, only a partial period TA (FIG. 2D) of its OFF period is extracted and fed into the optical spectrum analyzer 207 via the second optical path switch 33.
The optical spectrum analyzer 207 regards this incoming optical signal in the partial period TA of the OFF period in the amplified optical signal, as amplified spontaneous emission (ASE), and obtains the light intensity PASE (.lambda.=.lambda.1, .lambda.2, .lambda.3, . . . , .lambda.n-1, .lambda.n, . . . ) at each wavelength .lambda. of this amplified spontaneous emission.
The optical spectrum analyzer 207 sends the measured light intensity PASE(.lambda.) to the controller 208.
(Explanation of Probe Method)
The probe method measures the wavelength characteristics of an optical fiber amplifier in WDM transmission.
That is, this probe method measures the characteristics by using weak light (probe light) having no influence on the inversion distribution state of an optical fiber amplifier fixed by signal light (saturated signal light).
In this probe method, signal light having a single wave to several waves is used to set an optical fiber amplifier in the same state as WDM transmission.
FIG. 14 shows an evaluation method of measuring the wavelength characteristics of an optical fiber amplifier by using this probe method.
That is, as shown in FIG. 14, an optical multiplexer 303 multiplexes output signal light from a light source 301 and probe light emitted by a light source 302 via an optical attenuator 309.
(Measurement of Light Intensity PIN)
By setting optical path switches 304 and 306 in states indicated by the dotted lines in FIG. 14, the input light intensity PIN of the light source 302 with respect to an optical fiber amplifier 5 is obtained.
(Measurement of Light Intensity POUT)
By setting the optical path switches 304 and 306 in states indicated by the solid lines in FIG. 14, the amplified output light intensity POUT of the light source 302, which is output from the optical fiber amplifier 5, is obtained.
As shown in FIGS. 15A and 15B, from the optical spectrum of the amplified output light intensity POUT of the light source 302, PASE is obtained by the level interpolation method.
FIG. 15B is an enlarged view of an output light intensity POUT component enclosed with the circular dotted line in FIG. 15A.
(Combination of Conventional Pulse Method and Probe Method)
FIG. 16 depicts an evaluation apparatus for measuring the wavelength characteristics of an optical fiber amplifier by combining the conventional pulse method and probe method.
That is, as shown in FIG. 16, a light source 401 for outputting signal light and a light source 402 for outputting probe light are connected to optical attenuators 408 and 409, respectively.
An optical multiplexer 403 multiplexes the output signal light (FIG. 17A) from the optical attenuator 408 and the output probe light from the optical attenuator 409.
The output light from this optical attenuator 403 is input to an optical fiber amplifier 5 via an optical path switch 404.
Also, the output optical signal (FIG. 17B) from the optical fiber amplifier 5 is input to an optical spectrum analyzer 407 via an optical path switch 406.
(Measurement of Light Intensity PIN)
The light source 401 generates signal light (FIG. 17A) from an optical pulse modulated by a pulse from a pulse pattern generator 400.
The optical output from the light source 401 is shut off by a shutter of the optical attenuator 408, and the optical path switches 404 and 406 are set in states indicated by the dotted lines in FIG. 16. In this manner, the input light intensity PIN at each wavelength (.lambda.) of the light source 402 with respect to the optical fiber amplifier 5 is obtained.
(Measurement of Light Intensity POUT)
Next, the optical output from the light source 401 is transmitted by opening the shutter of the optical attenuator 408, and the optical path switches 404 and 406 are set in states indicated by the solid lines in FIG. 16. Consequently, the light intensity POUT of probe light in which modulation of the signal light shown in FIG. 17A is amplified at the central sampling point (FIG. 17B) in the OFF period is measured in the half period of the optical pulse in synchronism with the optical spectrum analyzer 407.
(Measurement of Light Intensity PASE)
In addition, by shutting off the optical output from the light source 402 in this state by the shutter of the optical attenuator 409, the light intensity PASE of amplified spontaneous emission, which is amplified at the moment the signal light modulation is turned off, is measured in the half period of the optical pulse in synchronism with the optical spectrum analyzer 407.
Unfortunately, the aforementioned conventional optical amplifier evaluating method and optical amplifier evaluating apparatus still have problems to be solved.
That is, the apparatus as shown in FIG. 12 poses no problem when measuring the wavelength characteristics of an optical amplifier in a predetermined range. To perform measurements in a broad range, however, it is necessary to prepare wavelength-multiplexed signal light throughout measurement wavelengths as shown in FIG. 13 and measure the gain and noise figure at each signal wavelength. This enlarges and complicates the apparatus.
In the apparatus using the conventional probe method as shown in FIG. 14, if the probe light wavelength and the signal light (saturated signal light) wavelength are close to each other, the spectra of these light components overlap each other.
If this is the case, the optical spectrum analyzer cannot measure the probe light component alone, so it is impossible to measure the characteristics in the vicinity of the signal light (saturated signal light) wavelength (1995 IEICE Communication Society Convention B-737 "Method of Measuring Gain of EDFA by Using Probe Light").
Furthermore, the apparatus combining the conventional pulse method and probe method as shown in FIG. 16 obtains a numerical value during a transient phenomenon of amplified spontaneous emission (ASE) a few ten .mu.s after the modulation of the signal light is turned off, as shown in FIG. 17D, rather than a numerical value immediately after the signal light modulation is turned off, as shown in FIG. 17C, owing to a low response speed of an amplifier of the optical spectrum analyzer 407 and a low modulation frequency of the light source (signal light).
In the apparatus combining the conventional pulse method and probe method, therefore, the light intensity of amplified spontaneous emission is measured at several points and extrapolated by linear approximation to a certain time immediately after the modulation of the signal light is turned off. This results in poor measurement accuracy.
That is, the apparatus combining the conventional pulse method and probe method shown in FIG. 16 has poor measurement accuracy for the reason to be explained below.
The optical spectrum analyzer 407 used in the conventional apparatus combining the pulse method and probe method shown in FIG. 16 measures in synchronism with the OFF period of the light source (signal light) 401.
Accordingly, the frequency band of an internal light-receiving amplifier of this optical spectrum analyzer 407 must be widened. This increases noise and deteriorates the measurement accuracy.
Also, during the ON period of the light source (signal light) 401, high power is input to the optical spectrum analyzer 407, so the internal light-receiving amplifier of the optical spectrum analyzer 407 is saturated.
A period Te immediately after the light source (signal light) 401 is turned off shown in FIG. 17C indicates a period in which the internal light-receiving amplifier of the optical spectrum analyzer 401 recovers from this saturated state. No normal measurements can be performed during this period Te (from about a few hundred .mu.s to about a few ms).
In FIG. 17C, reference symbol Tc denotes a period during which the light source (signal light) 401 keeps the same intensity as the ASE intensity in the ON period; and Td, a period during which the ASE intensity changes.
On the other hand, the response time (the time which rare earth element light of metastable erbium doped in the core of an optical fiber of an optical fiber amplifier requires to recover to the ground state) of the amplified spontaneous emission (ASE) of the optical fiber amplifier is also from about a few hundred .mu.s to a few ms.
Accordingly, during the period in which the internal light-receiving amplifier of the optical spectrum analyzer 401 recovers from the saturated state and becomes capable of measurements, the amplified spontaneous emission (ASE) of the optical fiber amplifier has changed its intensity from that during the ON period of the light source (signal light) 401.
Hence, as shown in FIG. 17D, the changing amplified spontaneous emission intensity is measured at several sampling points, and a true amplified spontaneous emission intensity P1 (the intensity of the amplified spontaneous emission ASE in the ON period of the signal light 401) is analogically measured by approximation. This analogy also deteriorates the measurement accuracy.